Catalytic Pyrolysis of Waste Polyethylene into Aromatics by H3PO4

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Catalysis and Kinetics

Catalytic Pyrolysis of Waste Polyethylene into Aromatics by H3PO4-activated Carbon Kai Sun, Qunxing Huang, Xiangdong Meng, Yong Chi, and Jianhua Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02091 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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Catalytic Pyrolysis of Waste Polyethylene into Aromatics by H3PO4-activated Carbon

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Kai Sun, Qunxing Huang*, Xiangdong Meng, Yong Chi, Jianhua Yan

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State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou310027,

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People’s Republic of China

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KEYWORDS: polyethylene; catalytic pyrolysis, oil, aromatics, H3PO4

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ABSTRACT: H3PO4-activated carbon was used as the catalyst in the pyrolysis of waste

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polyethylene (PE) to study its catalytic effect on the enrichment of the aromatics. The effect

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of the mass fraction ratio of phosphorus to wood chips (defined as the P/WC ratio) during

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impregnation and the residence time of the catalytic reaction on the product yield and the oil

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components were investigated and the catalytic mechanism was discussed. The increase of

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the P/WC ratio from 10 to 50% and the residence time from 1 to 5 s would increase the oil

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yield from 23.2 to 41.8% and 40.8%, respectively. Alkanes, alkenes and aromatics were the

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main components in the oils. The content of aromatics increased along with the residence

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time, and could be up to 30.0% when the P/WC ratio was 40%. Monocyclic aromatics,

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mainly alkyl and alkenyl benzenes with carbon atom numbers between 7 and 10 were the

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main aromatic compositions, the highest content of which was up to 23.8% when the P/WC

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ratio was 40% and the residence time was 3s. High P/WC ratio would accelerate the

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conversion of isomerized products to aromatized ones. The hydrogen transfer reactions 1 ACS Paragon Plus Environment

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catalyzed by Brønsted acid sites such as P-OH in C-O-PO3, C2-PO2, and C-PO3, and the

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direct dehydrogenation catalyzed by dehydrogenation active sites like P=O were the main

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aromatization processes.

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1. Introduction

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Nowadays, due to the characteristics of lightweight, easy-processing, and low-cost,

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plastics have been widely used in numerous sectors, such as construction, automotive,

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manufacturing, and medical.1, 2 It is reported that the production of plastics in the whole

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world has reached 322 million tons in 2015.3 The expanding demand for plastics products has

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provoked the production of large quantities of waste plastics, which has led to serious

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environmental and social problems4 due to their short life span and non-biodegradability.5-7

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Among all the polymers, polyethylene (PE) is the main components of waste plastics, which

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accounted for around 40% of the waste plastics.8 To date, mechanical recycling, landfill, and

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combustion are still the first options for waste plastics treatment,6 which are either costly and

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complicated, or detrimental to environment, or energy-wasting.9-11 Compared with these

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conventional disposal methods, pyrolysis, especially catalytic pyrolysis, has become a

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promising energy-recovery method for the conversion of waste plastics like PE to high

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value-added products, such as aromatics. 3, 10, 11

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Commonly used catalysts in the pyrolysis of plastics are mainly (metal impregnated)

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zeolite catalysts, transition metal catalysts, and conventional acid or alkaline solids,12,

13

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which could reduce the energy consumption, change the products distribution, and improve

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the oil quality.14-16 Marcilla et al.17 found that the content of aromatics in the oil could be up 2 ACS Paragon Plus Environment

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to 52.1% by HZSM-5 in the catalytic pyrolysis of LDPE at 500 °C. However, the oil yield

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was only 18.3%. Elordi et al.18 found that using HY zeolites in the pyrolysis of HDPE in a

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conical spouted bed reactor at 500 °C could increase the oil yield to around 72.6%, while the

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content of aromatics in oil was just around 24.1%. Aguado et al.2 comparatively studied the

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catalytic pyrolysis of LDPE by mesostructured zeolites Al-MCM-41, and found that the oil

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yield could reach 42 % compared with 26% achieved by HZSM-5 while the aromatic yield

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was only 4% in the gasoline fraction. Elordi et al.19 obtained an oil yield as high as 61.6%

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when spent FCC was used as the catalyst in the pyrolysis of HDPE at 500 °C, while the

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aromatics only accounted for 11.8% of the gasoline fraction. Mosio-Mosiewski et al.20

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obtained an oil yield of 64.6% and an aromatic yield of 18.3% using amorphous acid

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aluminosilicate catalyst in the high pressure (10Mpa) catalytic cracking of PE at 420 °C.

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Renzini et al.21 found that the content of aromatics in the cracking oil of PE catalyzed by

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Zn-ZSM-11 at 500 °C could be up to 53.0% and the oil yield could reach 55.1%. Uemichi et

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al.22, 23 obtained a high content of aromatics which could be up to 46.2 and 30% when using

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Pt-activated carbon in the catalytic pyrolysis of PE and PP, respectively. However,

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serious-coking and high-cost limit the promotion of these transition metal catalysts.

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Nowadays, cost-efficient carbon-based catalysts are gaining attention due to their

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low-cost, controllable pore size, and adjustable surface properties.11 The chemical activated

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carbon was one of the cheap and widely used carbonaceous materials which was endowed

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with catalytic activities due to the surface modification of activating agents like H3PO4. The

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H3PO4-activated carbon has been used as the catalyst for the oxygen reduction reaction in the

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air-cathode microbial fuel cells,24 the selective dimerization of isobutene,25 and the catalytic

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fast pyrolysis of cellulose and biomass,

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H3PO4-activated carbon as the catalyst in the pyrolysis of plastics and production of

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etc. But as far as we know, few researchers used

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aromatics.

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In our previous work, the catalytic effect of H3PO4-activated carbon to the pyrolysis oil

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of mixed plastics (PE, PP and PS) was investigated and it showed high selectivity towards

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aromatics (66.0%).11 However, the main factors affecting the catalytic process and the

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reaction mechanisms haven’t been discussed detailedly. In this study, only the waste PE was

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chosen as the feedstock to avoid the influence of the interaction among different plastic

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components. The effect of the impregnation ratio in activation and the residence time in

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catalytic reaction to the oil quality were analyzed, and the role of the catalytic active sites

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were discussed, which could provide a better understanding of the catalytic behavior of

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H3PO4-activated carbon in the pyrolysis of plastics.

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2. Materials and Methods

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2.1. Materials

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Polyethylene (PE) powder was purchased from Guanbu Electromechanical Technology

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Company (Shanghai, China). After being received, it was dried and sieved to a particle size of

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smaller than 0.18 µm.

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Catalysts used in this work were activated carbons prepared by chemical activation

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methods. The H3PO4-activated carbon (P-AC) was prepared by impregnating wood chips by

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H3PO4 solution for 24h. The mass fraction ratios of P to wood chips (abbreviated as P/WC

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ratio) were 10, 20, 30, 40, and 50%, respectively. After impregnation, the mixtures were dried

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and calcined from 50 to 600 °C under nitrogen flow at a heating rate of 10°C/min and with a 4 ACS Paragon Plus Environment

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dwell time of 90 min. After being cooled, they were washed, dried and ground to 80-200

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mesh. These catalysts were marked as P-AC-10% - 50% according to the different P

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concentrations in the impregnating solution correspondingly.

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In order to figure out the action mechanism of H3PO4, HNO3 and other typical

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phosphorus compounds such as (NH4)2HPO4 were also used as the impregnating agents to

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prepare activated carbon (abbreviated as HNO3-AC, (NH4)2HPO4-AC, respectively). The

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HNO3 solution used to prepare HNO3-AC had the same pH with the H3PO4 solution used to

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prepare P-AC-30%. While the (NH4)2HPO4 solution had the same P/WC ratio with

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P-AC-30%.

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2.2. Experimental Setup and Product Analysis

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The catalytic pyrolysis of PE was carried out on a two staged fixed bed reactor equipped

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with a temperature-programmed device. The quartz boat with 10 g of PE sample in it was

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placed at the first stage, while a mixture of 4 g of catalysts and quartz sand was loaded at the

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second stage and supported by the silica wool. The first stage was heated from 50 to 550 °C

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at a heating rate of 20 °C/min and with a dwell time of 60 min, while the second stage was

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kept at a constant temperature of 600 °C. The residence time was determined by the flow rate

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of the nitrogen, which was defined as Eq. (1)

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Residencetime =

Vc × Ta × 60 FN 2 × Tc

(1)

where is the volume of the mixture of catalyst and quartz sand, dm3; is the gas 5 ACS Paragon Plus Environment

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flow rate of nitrogen at ambient temperature, L/min; is the ambient temperature, K; is

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the catalytic temperature, K.

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By calculation, 0.518, 0.258, 0.173, 0.129, and 0.104 L/min at ambient temperature

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corresponded to 1.0, 2.0, 3.0, 4.0, and 5.0 s, respectively. The addition of quartz sand was to

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ensure the consistency of the mixture volume, thus when the gas flow rate was constant, the

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residence times among different groups could maintain the same. The schematic diagram

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illustrating the principle of the catalytic pyrolysis is shown in Fig. 1.

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The pyrolytic vapors were cooled down and collected by three gas washing bottles filled

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with dichloromethane. The circulating water was used to keep the gas washing bottles cold.

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After reaction, the dichloromethane solution of oil in gas washing bottles was dried at 40 °C

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and weighted, which was considered to be the mass of the oil product. The mass differences

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of the quartz boat and the second stage were considered to be the mass of the solid residues

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and the coke on the catalysts, respectively. The gas yield was calculated by mass balance. Gas

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chromatography-mass spectrometry (GC-MS, Agilent 6890-5973, USA) with a DB-5 column

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was used to analyze the main 40 - 50 compositions of the oil product. The oven was held at

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80 °C for 2 min, then heated to 250 °C at a rate of 20 °C/min, and kept for 10 min for each

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test. Semi-quantitative analysis was used to calculate the selectivities of the oil compositions

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based on the peak areas in the chromatogram, and the content was expressed by area %.

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A scanning electron microscopy coupled with energy-dispersive spectroscopy

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(SEM-EDS, SIRON, FEI, Holland) was used to characterize the surface morphology and

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actual P content on the surface of the catalysts. 6 ACS Paragon Plus Environment

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An automatic specific surface and micropore size analyzer (AUTOSORB-IQ2-MP,

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Quantachrome, USA) was used to measure the specific surface area and the pore size

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distribution of the catalysts. The specific surface area was calculated by multiple Brunauer–

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Emmett–Teller (BET) method. The information of micropore and mesopore was obtained by

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HK and BJH method, respectively.

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The surface chemical structure of the catalysts was recorded by an X-ray photoelectron

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spectrophotometer (XPS, VG ESCALAB MKII, VC, UK). High-resolution spectra of P 2p

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was recorded using Non-monochromatic Mg Kα X radiations (hγ=1253.6 eV) and

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curve-fitted by multiple Gaussian function.

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3. Results and discussion

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3.1. Characteristics of the Catalysts.

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The SEM pictures of the catalysts P-AC-10% - 50% are shown in Fig. 2. It could be

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seen that no apparent pores were formed on the surface of P-AC-10% and P-AC-20%. When

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P concentration in impregnating solution was high than 30%, the activation effect of

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phosphoric acid formed developed pore structures on the carbon surfaces. P-AC-40% had the

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most developed cylindrical pores along the woody fibers. It could also be seen that the pores

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of P-AC-50% were apparently blocked probably because of the phosphorus pentoxide formed

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by the decomposition of excessive phosphoric acid and the collapsed pore walls.

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The specific surface area and the pore properties of the P-ACs and the P content on the

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catalyst surfaces are listed in Table 1. P content on the catalysts increased along with the 7 ACS Paragon Plus Environment

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concentration of impregnating solution, which could reached 25 wt% when the P/WC ratio

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was 50%. The maximum BET surface area and total pore volumn could be up to 742.9 m2/g

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and 0.860 cm3/g, respectively, when the P/WC ratio reached 40%. The average pore size of

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P-AC-40% was 5.00 nm, which indicated that it was a developed mesoporous activated

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carbon. When the P/WC ratio continued to grow, the specific surface area and the total pore

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volume decreased, which was caused by the blockage showed in Fig. 2(e). While the increase

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of the average pore diameter from 5.00 to 5.79 nm indicated that there may be the collapse of

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the pore walls and the merge of the small pores.

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3.2. Properties of the Oil Product at different P/WC Ratios

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The product yields of the groups with different P/WC ratios and the same residence time

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(Group 0# -5#) were listed in Table 2. The mass of the solid residue derived from PE

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pyrolyzed at 550 °C could be negligible. The quartz sand was considered to be a reference

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material in the non-catalytic pyrolysis group. It could be observed that the gas was the main

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products while the oil yield was only 23.2% in the non-catalytic pyrolysis group (0#). The

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slight weight increment of quartz sand was probably due to the attachment of the heavy

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long-chain hydrocarbons on its surface. When the catalysts were used, the oil and coke yield

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increased with the P/WC ratio, which was probably due to the polycondensation of light

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hydrocarbons into heavier molecules with higher solubility in dichloromethane,27 and the

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further condensation of large molecules (such as polyaromatics) into coke catalyzed by acid

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sites.

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The components of oil products catalyzed by P-ACs with different P/WC ratios are 8 ACS Paragon Plus Environment

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shown in Fig. 3(a). The main components in the oil products were alkanes, alkenes, and

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aromatics. The non-catalytic pyrolysis yield 74.4% alkenes (including monoenes and dienes)

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and 24.1% alkanes with a wide boiling point range.28 The addition of P-AC significantly

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increased the content of aromatics and decreased the alkene yield. When the P/WC ratio was

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below 20%, aromatization process was not obvious. The content of alkenes only decreased

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slightly probably due to the dehydrogenation reaction catalyzed by the dehydrogenation sites

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such as C=O and P=O,29 and a small amount of aromatics were thus formed. When the P/WC

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ratio reached 30%, the content of alkanes and aromatics significantly increased at the cost of

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a great quantity of alkenes. That was due to the hydrogen transfer reactions of alkenes which

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yielded H-rich materials like alkanes and H-deficient materials like aromatics.30 The

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increasing number of acid sites could enhance the transformation of alkenes to carbonium

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ions, which provided large quantities of feedstocks for the secondary reactions such as

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hydrogen transfer reaction. This was detailedly explained in Fig. 9. When the P/WC ratio

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continued to grow, the content of alkenes was no longer significantly reduced. The content of

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alkanes in group 4# catalyzed by P-AC-40% decreased to 24.6% and the content of aromatics

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reached 30.0%. This was due to the promotion of dehydrogenation which converted alkenes

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to aromatics and alkanes to both alkenes and aromatics. This process was catalyzed by the

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increasing number of dehydrogenation active sites like P=O caused by the increasing

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concentration of the impregnating solution, and the detailed catalytic mechanisms are shown

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in Fig. 9. The components of the oil in group 5# were similar to group 4#. The aromatic yield

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was slightly decreased owing to the decline of the catalytic efficiency. The decreased specific

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surface area caused by the blockage and merge of the pores would reduce the number of the 9 ACS Paragon Plus Environment

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acid sites on the catalyst surface, which led to the decline of the catalytic efficiency.

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The isomerization and aromatization degrees of the main oil components are shown in

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Fig. 4(a). It could be concluded that the total content of isomerized and aromatized products

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increased along with the P/WC ratio in general. When the P/WC ratio was lower than 20%,

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the growth of isomerization products was much faster than that of aromatized products.

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However, when the P/WC ratio was higher than 30%, the content of aromatized products

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significantly increased, while the content of isomerized products gradually decreased. When

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P/WC ratio was higher than 40%, the reduction of catalytic efficiency caused by the decrease

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of the specific surface area started to inhibit the aromatization reaction. However, the further

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decrease in the content of isomerized products was still remarkable, which was because the

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surface acidity was still strong enough for the transformation of isomerized products to

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aromatized ones.

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It was believed that the catalytic pyrolysis of PE by solid acid catalysts followed the

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carbocation mechanism.31, 32 The straight-chain hydrocarbons (such as normal alkanes and

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1-alkenes) derived from the thermal pyrolysis of PE formed secondary carbenium ions on the

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acid sites firstly, which could either form stable tertiary carbenium ions, and then formed the

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isomerized products, or crack into smaller alkenes and formed new carbenium ions by

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β-scission.33, 34 When the P/WC ratio was relatively low, the increase in the number of the

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acid sites and the dehydrogenation active sites accelerated the formation of carbenium ions,

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which led to the promotion of isomerization reaction and β-scission. While the impregnation

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by higher concentration of H3PO4 increased the acidity and the number of the acid sites on 10 ACS Paragon Plus Environment

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P-AC, which promoted the aromatization activity, such as the hydrogen transfer reactions and

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dehydrogenation of the already formed carbenium ions. Thus, a large quantity of aromatics

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formed at the cost of tertiary carbenium ions, leading to the reduction in the content of the

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isomerized products.

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The GC-MS chromatograms of the oil products catalyzed by the P-AC under different

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P/WC ratios were exhibited in Fig. 5(a). It could be seen that C7 – C23 components were the

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main components in the catalytic pyrolysis oils for all groups. With the increase of the P/WC

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ratio, the retention time of the peak with high intensity gradually decreased, which indicated

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that the oil products were getting lighter. This was probably due to the scission of long-chain

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hydrocarbons into shorter ones enhanced by the increase in the number of the acid sites. The

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number and intensities of the peaks representing aromatics increased along with the

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impregnation concentration of P. The presence of aromatic 10, 13, 15, and 22 showed the

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trend of dehydrocyclization during aromatization process, which appeared in all groups.

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While aromatic 11, 20, 21 were probably derived from the aromatization of the isomerized

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products, which did not appear until the P/WC ratio was higher than 30%. This also proved

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that the strong surface acidity and high P concentration promoted the transfer of isomerized

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products to aromatized ones.

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The effect of different P/WC ratios on the aromatic compositions classified by the

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number of aromatic rings are shown in Fig. 6(a). The content of main aromatic compositions

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is listed in Table S1. Monocyclic aromatic hydrocarbons (MAHs) were the dominant

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components in aromatics for all groups, followed by bicyclic aromatic hydrocarbons (BAHs) 11 ACS Paragon Plus Environment

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and a small amount of tricyclic aromatic hydrocarbons (TAHs). MAHs were mainly alkyl and

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alkenyl benzenes with carbon atom numbers between 7 and 10. BAHs were mainly

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naphthalenes and a small amount of biphenyl. The fused-ring aromatics such as anthracene

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and penanthrene were the main compositions of TAHs. The branches on MAHs and BAHs

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may be formed by both the cyclization of the long-chain carbonium ions and the alkylation of

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the already formed aromatics catalyzed by the acid sites.35, 36 Acid sites were effective in

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transforming alkanes or alkenes into long-chain carbonium ions and aromatics into

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arylcarbenium ions, which were the feedstocks in the cyclization and alkylation reaction,

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respectively. For the MAHs, the content increased along with the P/WC ratio and reached the

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peak (23.78%) when the P/WC ratio was around 40%. After that the content of MAHs

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decreased to 17.97%, which was due to the promoted condensation reaction of the MAHs

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into polycyclic aromatics catalyzed by the strong acid sites. Aromatics like MAHs could form

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arylcarbenium ions on Brønsted acid sites, which could yield polyaromatics by continuous

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dehydrogenation and condensation reaction. For the BAHs and TAHs, the content also

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increased as the P/WC ratio increased but with a much lower growth rate compared with

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MAHs. What’s more, when the P/WC ratio was lower than 30%, there were no TAHs

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detected. Considering the selectivities toward different aromatic compositions, a P/WC ratio

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of 40% was appropriate for the production of aromatics, especially MAHs.

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3.3. Properties of the Oil Product under Different Residence Times

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Group 3# and 6# - 9# in Table 1 exhibited the effect of different residence times on the

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product yields. As the residence time increased, the oil yield and coke yield increased, which


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was due to the polymerization of the light products into heavier ones. When PE was catalyzed

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by P-AC-30% with the residence time of 5 s (group 9#), the oil yield could be up to 40.8%,

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which was similar to 41.8% obtained by group 5# catalyzed by P-AC-50% with the residence

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time of 3 s. However, the coke yield in group 9# was only 59.4% of that in group 5#. This

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indicated that prolonging residence time was more suitable and efficient in improving the

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product quality.

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The components of the oil products catalyzed by P-ACs under different residence times

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are shown in Fig. 3(b). When the residence time was less than 2 s, the aromatization reaction

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was slow and the main components in the oils were alkenes just like the oils catalyzed by

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P-AC with low P/WC ratios. Hydrogen transfer reactions between alkenes firstly formed

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H-deficient intermediates like dienes and cycloalkenes. Due to the limit of the number of the

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acid sites, few intermediates could be converted into aromatics by further intermolecular

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hydrogen transfer.37 During this stage, aromatics may be mainly formed by direct

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dehydrogenation of alkanes and alkenes, which was catalyzed by the dehydrogenation active

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sites. When the residence time was prolonged to 3 s, the content of alkanes and aromatics

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increased at the cost of alkenes. The already formed H-deficient intermediates started to be

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transformed to aromatics, and lots of alkanes were produced as the H-rich products. As the

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residence time was prolonged from 3 s to 5 s, the consumption of H-deficient intermediates

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weakened the aromatic produced from hydrogen transfer reactions. Direct dehydrogenation

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of alkanes and alkenes led to the decrease of alkane content and the slight increase of

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aromatic content. These parts of catalytic reaction mechanisms were summarized in Fig. 9.

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Compared with using P-AC with high impregnation ratio, prolonging residence time was less 13 ACS Paragon Plus Environment

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efficient in increasing the yield of aromatics.

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Fig. 4(b) showed the changes in the isomerization and aromatization degrees of the oil

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products under different residence times. P-AC-30% provided a moderate acidity and an

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appropriate number of acid sites, leading to the simultaneous increase in the content of both

277

isomerized and aromatized products (see Fig. 9). However, the conversion of isomerized

278

products to aromatized ones still existed, which could be proved by the slightly decreased in

279

the content of isomerized products at a residence time of 5 s.

280

The GC-MS chromatograms of the oil products obtained under different residence times

281

are exhibited in Fig. 5(b). As the residence time increased, the heavy long-chain

282

hydrocarbons were converted into lighter ones. The intensity of the peaks representing

283

aromatics was growing and the main aromatic species were similar to those in the oils

284

catalyzed by different P-ACs. From Fig. 6(b) and Table S2 it could be seen that the content of

285

MAHs was rapidly increased from 3.14% to 21.07% when the residence time was prolonged

286

from 1 s to 5 s. While for the BAHs, the content reached its peak (4.66%) when the residence

287

time was 3 s. After that, the content of BAHs gradually decreased with the increase of the

288

residence time. This was due to the condensation reaction of BAHs into heavier fused-ring

289

aromatics catalyzed by the strong acid sites, which was difficult to be collected and detected

290

by the GC-MS.

291

3.4. Discussion of the Catalytic Mechanism

292

P-AC-30% was used to study the effect of H3PO4 on PE conversion. Meanwhile 14 ACS Paragon Plus Environment

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(NH4)2PO4 and HNO3 were also used as the activating agents for comparative studies. The oil

294

components catalyzed by P-AC, (NH4)2PO4-AC, and HNO3-AC are shown in Fig. 7.

295

The oil obtained by the catalysis of (NH4)2PO4-AC had a higher content of alkenes and a

296

lower content of aromatics and alkanes, which were probably due to the weakened hydrogen

297

transfer reactions catalyzed by acid sites. The formation of acid sites was strongly related to

298

the properties of the impregnating solution. Compared with the strong acid impregnating

299

solution (pH=1.45) of P-AC-30%, the solution used for the impregnation of (NH4)2PO4-AC

300

was weakly alkaline (pH=8.4) which was detrimental to the formation of acid sites on the

301

activated carbon. Thus, aromatics may be mainly formed by the direct dehydrogenation

302

catalyzed by some dehydrogenation active sites. It could be confirmed that the hydrogen

303

transfer reactions catalyzed by acid sites was one of the important sources of aromatics, and

304

enough acidity of the impregnating solution was necessary.

305

The selectivity of HNO3-AC to aromatics was 16.8%, which was slight higher than that

306

of P-AC-30%. However, the content of other compositions, mainly nitrogenous and

307

oxygenated compounds was as high as 63.9 %, which would greatly reduce the quality of the

308

oil. These compounds were probably derived from the reactions between the feedstocks and

309

the unstable nitrogen- and oxygen-containing functional groups, which was formed during

310

the HNO3 impregnation. In comparison, phosphorus complexes formed during H3PO4

311

activation were insoluble and thermally stable at even 700 °C on the carbon surface,38, 39

312

which had little negative effect on the oil quality. Thus, H3PO4 was more suitable for the

313

production of acid catalyst compared with other common volatile acid. 15 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

314

Fig. 8 illustrates the high resolution P 2p spectra and the curve-fitting results.38, 40 It

315

could be seen that C3-PO was the main phosphorus-containing group, the content of which

316

was up to 51.5%, followed by C-O-PO3 (28.5%) and P2O5 (11.5%). P2O5 was produced from

317

the thermal decomposition of H3PO4 during the activation, the content of which has been

318

greatly reduced due to its vaporization above 580 °C.41

319

P-OH in C-O-PO3, C2-PO2, and C-PO3 may be the main Brønsted acid sites formed by

320

H3PO4 treatment.42 These acid active sites were responsible for the hydrogen transfer

321

reactions, cyclization and alkylation of the aromatics. Hydrogen transfer reaction was a main

322

source of aromatics. However, the production of aromatics through hydrogen transfer

323

reactions would inevitably yield a large quantity of alkanes, which inhibited the improvement

324

of the aromatic selectivity.43 It was reported that P=O played an important role in the catalytic

325

dehydrogenation reaction not only because it could enhance the dehydrogenation activity of

326

other active sites such as C=O, but also due to its ability to be independent active sites for

327

dehydrogenation.29 Thus, the direct dehydrogenation catalyzed by P=O in C-O-PO3, C2-PO2,

328

C-PO3 and C3-PO was another important way of aromatic formation which consumed both

329

alkanes and alkenes. The reaction mechanisms of catalytic aromatization are illustrated in Fig.

330

9.

331

4. Conclusion

332

H3PO4-activated carbon (P-AC) was used as the catalyst in the pyrolysis of PE carried

333

out in a two staged fixed bed reactor in order to study its effect on the aromatic enrichment in

334

pyrolysis oil. The effect of the P/WC ratio and residence time was investigated and the 16 ACS Paragon Plus Environment

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catalytic mechanism was discussed. The main conclusions can be drawn as follows:

336

(1) The increase of the P/WC ratio (10 to 50%) and residence time (1 to 5 s) led to the

337

increase in the oil yield from 23.2% to 41.8% and 40.8%, respectively. However,

338

compared with prolonging residence time, increasing the P/WC ratio would lead to more

339

serious coking.

340

(2) The total content of the isomerized and aromatized products increased along with the

341

residence time and the P/WC ratio in general, while increasing the P/WC ratio would

342

accelerate the conversion of isomerized products to aromatized ones.

343

(3) The content of aromatics reached the peak (30.0%) when the P/WC ratio was 40%, while

344

the aromatic yield increased along with the residence time when it was between 1 s and 5

345

s. MAHs, such as alkyl and alkenyl benzenes with carbon atom numbers between 7 and

346

10, were the main components in aromatics, which could be up to 23.8% in the oil when

347

the P/WC ratio was 40% and the residence time was 3 s.

348

(4) H3PO4 treatment formed phosphorus-containing functional groups on the carbon surface,

349

which played a major catalytic role in the aromatic enrichment. The hydrogen transfer

350

reactions catalyzed by Brønsted acid sites such as P-OH in C-O-PO3, C2-PO2, and C-PO3,

351

and the direct dehydrogenation catalyzed by dehydrogenation active sites like P=O were

352

the main processes of aromatization.

353


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 48

354

Associated content

355

Supporting Information

356

The main aromatic compositions of the oils catalyzed by P-ACs with different P/WC ratios

357

(based on peak area). (Table S1)

358

The main aromatic compositions of the oils catalyzed by P-ACs with different residence

359

times (based on peak area). (Table S2)

360

Author information

361

Corresponding Author

362

*Telephone: +86-571-87952834. Fax: +86-571-87952438. E-mail: [email protected]

363

Notes

364

The authors declare no competing financial interest.

365

Acknowledgments

366

This work was supported by National Natural Science Foundation of China (Grant No.

367

51621005),

the

National

Key

Research

and

Development

Program

of

China

368

(2016YFE0202000), the Environmental Protection Special Funds for Public Welfare

369

(201509013) and the Fundamental Research Funds for the Central Universities.

370


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thermal and catalytic pyrolysis of plastic solid waste (PSW). Journal of environmental

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management 2017, 197, 177-198.

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of polyethylene in a two-step thermo-catalytic reaction system. Journal of Analytical &

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of waste polystyrene and high-density polyethylene using spent FCC catalyst. Polymer

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of plastic wastes. Energy Conversion & Management 2016, 115, 308-326.

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the valorization of waste polyolefinic plastics to produce fuels and chemicals. A review.

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hydrocarbons over metal-supported activated carbon catalysts. Journal of Analytical &

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hydrocarbons over Pt- and Fe-containing activated carbon catalysts. Journal of Analytical &

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of P-doped activated carbon as a catalyst for air-cathode microbial fuel cells. Journal of

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carbon catalyst. Acs Sustainable Chemistry & Engineering 2017, 5, (11).

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hydrocarbons from municipal plastic wastes. Resources Conservation & Recycling 1998, 23,

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Aluminosilicate of the MCM-41 Type: Its Catalytic Activity in n -Hexane Isomerization.

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Liquids as Lewis Acid Catalysts for the Alkylation of Aromatic Compounds with Dodecene.

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164, (14), 135-144.

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495

Tables

496

Table 1. Physicochemical properties of the different P-ACs.

Page 24 of 48

P content

BET surface area

Total pore volume

Average pore

(wt%)

(m2/g)

(cm3/g)

diameter (nm)

P-AC-10%

9.1

18.7

0.022

17.26

P-AC-20%

11.9

21.0

0.053

13.77

P-AC-30%

18.0

62.2

0.083

5.94

P-AC-40%

22.2

742.9

0.860

5.00

P-AC-50%

25.0

630.1

0.851

5.79

Sample

497 498


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499

Energy & Fuels

Table 2. Product yields under different P/WC ratios and residence times Group P/WC ratio

Residence

time Oil yield (%)

(s)

Coke

yield Gas yield (%)

(%)

0#

Quartz sand 3s

23.2±0.2%

0.2±0.1%

76.8

1#

10%

3s

27.0±0.8%

1.2±0.5%

71.8

2#

20%

3s

31.2±1.1%

2.3±0.2%

66.5

3#

30%

3s

35.1±0.6%

3.7±0.3%

61.2

4#

40%

3s

37.5±1.7%

6.1±0.4%

56.4

5#

50%

3s

41.8±1.4%

7.7±0.3%

50.5

6#

30%

1s

28.4±0.9%

1.6±0.4%

70.0

7#

30%

2s

31.3±0.8%

2.5±0.2%

66.2

8#

30%

4s

40.6±1.6%

4.1±0.8%

55.3

9#

30%

5s

40.8±1.4%

4.6±1.2%

54.6

500 501


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

502

Figures

503

Figure 1. The schematic layout of the experimental device.

Page 26 of 48

504 505


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506 507

Energy & Fuels

Figure 2. SEM pictures of (a) P-AC-10%, (b) P-AC-20%, (c) P-AC-30%, (d) P-AC-40%, and (e) P-AC-50%.

508 509


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

510 511

Page 28 of 48

Figure 3. The components of oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different residence times.

512 513

(a)

514 515 516

(b)


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517 518

519 520

Energy & Fuels

Figure 4. The isomerization and aromatization degrees of the oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different residence times.

(a)

521 522 523

(b)


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524 525

Page 30 of 48

Figure 5. GC-MS chromatograms of oil products catalyzed by P-ACs at (a) different P/WC ratios and (b) different residence times.

526 527

(a)


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528 529 530

Energy & Fuels

(b)


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 48

532

Figure 6. The main aromatic compositions classified by the number of aromatic rings in the oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different

533

residence times.

531

534 535

(a)

536 537 538

(b)


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539 540

Energy & Fuels

Figure 7. The components of the oil products catalyzed by activated carbon modified by H3PO4, (NH4)2PO4, and HNO3.

541 542


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543

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Figure 8. High resolution P 2p XPS spectra of P-AC-30%.

544 545


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Energy & Fuels

Figure 9. The main catalytic mechanism of PE pyrolysis catalyzed by P-AC.

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Figure 1. The schematic layout of the experimental device. 44x12mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Energy & Fuels

Figure 2. SEM pictures of (a) P-AC-10%, (b) P-AC-20%, (c) P-AC-30%, (d) P-AC-40%, and (e) P-AC-50%. 172x198mm (300 x 300 DPI)


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Figure 3. The components of oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different residence times. 72x61mm (300 x 300 DPI)


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Energy & Fuels

Figure 3. The components of oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different residence times. 72x61mm (300 x 300 DPI)


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Figure 4. The isomerization and aromatization degrees of the oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different residence times. 72x61mm (300 x 300 DPI)


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Energy & Fuels

Figure 4. The isomerization and aromatization degrees of the oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different residence times. 72x61mm (300 x 300 DPI)


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Figure 5. GC-MS chromatograms of oil products catalyzed by P-ACs at (a) different P/WC ratios and (b) different residence times. 149x196mm (300 x 300 DPI)


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Energy & Fuels

Figure 5. GC-MS chromatograms of oil products catalyzed by P-ACs at (a) different P/WC ratios and (b) different residence times. 148x195mm (300 x 300 DPI)


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Figure 6. The main aromatic compositions classified by the number of aromatic rings in the oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different residence times. 72x61mm (300 x 300 DPI)


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Energy & Fuels

Figure 6. The main aromatic compositions classified by the number of aromatic rings in the oil products catalyzed by P-ACs under (a) different P/WC ratios and (b) different residence times. 72x61mm (300 x 300 DPI)


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Figure 7. The components of the oil products catalyzed by activated carbon modified by H3PO4, (NH4)2PO4, and HNO3. 72x61mm (300 x 300 DPI)


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Energy & Fuels

Figure 8. High resolution P 2p XPS spectra of P-AC-30%. 76x48mm (300 x 300 DPI)


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Figure 9. The main catalytic mechanism of PE pyrolysis catalyzed by P-AC. 150x170mm (300 x 300 DPI)


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Catalytic Pyrolysis of Waste Polyethylene into Aromatics by H3PO4

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